Beyond The End - The New End

Resume the lobby island placement after a restart

makeInitialIsland queues the island as thousands of small tasks (4 per
tick, several minutes in total) and sets isPlaced right away, but the
queue is not saved. Leaving the world before it finished left the
island with holes forever.

Count the executed tasks in the already saved (and unused) stepIndex
and save a new placementDone flag. On a world with isPlaced but not
placementDone, rebuild the same task list (same templates, same order)
and skip the tasks already done. Worlds saved before this change have
no placementDone and are treated as finished, so nothing is rebuilt in
them. StructureWarmup now preloads the templates unless the island is
complete (isComplete()), so a resume does not read them on the tick.